Embodiments of the invention relate generally to fluid-attenuated inversion recovery (FLAIR) imaging and, more particularly, to a system and method for reducing the T1 contribution to FLAIR imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
T2-weighted contrast is routinely used in current clinical practice for brain MR imaging. It aids in the diagnosis of diseases that have lesions with prolonged T2 compared to its surrounding tissues. Lesions frequently appear in white matter (WM) or gray matter (GM) adjacent to cerebrospinal fluid (CSF). In such cases, the lesions may be obscured by the bright signal of CSF on T2-weighted images. To improve the conspicuity of these lesions, fluid-attenuated inversion recovery (FLAIR) is commonly used to suppress CSF. However, the inversion recovery (IR) pulse used in FLAIR typically imparts T1 weighting that can decrease detectability and may lead to mischaracterization of some lesions. For example, FLAIR imaging provides highest sensitivity in the detection of lesions close to CSF such as the juxta-cortical and periventricular WM but is less sensitive in the posterior fossa. Frequently, both pure T2 and FLAIR images are acquired in clinical protocols.
T1 weighting is unavoidable in FLAIR. The IR pulse in the FLAIR sequence inverts the longitudinal magnetization (Mz) of all tissues, which then recover to their equilibrium magnetization (M0) based on their longitudinal relaxation times (T1). Due to the prolonged T1 of lesions compared to their surrounding tissue, lesions have reduced Mz compared with normal tissue at the time of excitation. This forces the use of long echo time (TE) to establish the T2-weighted contrast required to detect the long T2 lesions. Additionally, the T1 weighting is more pronounced at shorter repetition times (TR). It has been previously emphasized that long TRs and long TEs with FLAIR are preferable for detection of multiple sclerosis (MS) lesions.
Double IR (DIR) preparations allow the highlighting of certain types of brain lesions. The two IR pulses in a typical DIR preparation are timed to suppress two different types of tissues, for example, WM and CSF. While this produces an image with high contrast between WM and lesion, the contrast is entirely due to the T1 differences. Thus, DIR images produce even stronger T1 contrast than FLAIR.
It would therefore be desirable to have a system and method capable of minimizing T1 weighting and of producing images with pure T2 weighting over the range of T1s typical of brain tissue, white simultaneously suppressing CSF.